The Vertical Speed Indicator — Page 74, Lesson 90

Let’s pick up with the Vertical Speed Indicator, and I want to start with a critical operational warning that applies to the whole instrument. If the static line or the vent feeding this instrument becomes blocked, the needle will return to zero. That sounds reassuring, but it’s actually a trap — a blocked static supply means the instrument is no longer sensing the true static pressure, so a zero reading does not mean you’re in level flight. And here’s the bigger point: if the air supply to this instrument is blocked, it’s probable that the other pressure instruments — the Airspeed Indicator, the Altimeter, and the Machmeter — will also be affected, because they all share the same static source. So a VSI reading zero in a climb or descent should immediately make you suspect a static blockage, not a calm atmosphere. Now, the main problem with a standard VSI is lag — it takes time for the metering restriction to build up the differential pressure that drives the needle. To overcome that lag, we have the Instantaneous Vertical Speed Indicator, the IVSI. The IVSI incorporates an accelerometer unit, sometimes called a dashpot or a dynamic vane, which responds quickly to a change of altitude. Let me walk you through how it works during the start of a descent. Inside the instrument there’s a vertical acceleration pump — that’s the dashpot — containing a piston in a cylinder. When the descent begins, the piston immediately rises in the cylinder. That rise causes a temporary increase of pressure inside the capsule. The capsule expands, and the pointer gives an instant indication of descent — before the normal metering system has even caught up. Then, as the initial acceleration settles into a steady rate of descent, the piston slowly descends back to its original position. But by that time, the correct differential pressure between the capsule and the case has been set up through the main metering restriction, so the correct rate of descent continues to be shown. In other words, the dashpot gives you the instant response, and the metering restriction takes over for the steady-state reading. Now, the IVSI has errors peculiar to it. Because the dashpot assembly is so sensitive, the instrument tends to overreact in turbulent flying conditions — so small fluctuations should be ignored. And there’s a specific trap: in a steep, level turn, the piston will tend to sink towards the bottom of the cylinder, which produces a false indication of a climb. So in a steep turn, don’t trust the IVSI for your vertical speed. Let’s talk about presentation. Two types of scale are available: a linear scale and a logarithmic scale. The logarithmic scale is more easily read at the lower rates of climb and descent, because the graduations are spread out where you need the resolution. The linear scale spreads the same range evenly. Two construction notes: diaphragm overload stops may be fitted to prevent damage to the instrument if the rate of climb or descent exceeds the maximum to which the instrument is calibrated. And on some instruments, a zeroing screw is fitted so you can adjust the zero point. Finally, the serviceability checks. On the ground, the instrument should read zero, or the error should be within permissible limits: plus or minus 200 feet per minute at temperatures between minus 20°C and plus 50°C, and plus or minus 300 feet per minute outside those temperatures. There should also be no apparent damage to the instrument. In the air, you can check the accuracy of the instrument against the altimeter and a stopwatch during a steady climb or descent. And the instrument should indicate zero climb or descent when you’re in level flight. So the key mental model: the VSI is a rate-of-change instrument driven by static pressure differential, the IVSI adds a dashpot for instant response, and a zero reading with a suspected static blockage is a red flag, not a comfort.